1. Field of the Invention
The present invention relates to a mobile communication terminal equipped with a multipath interference canceller.
2. Related Background Art
In recent years, the rapid spread of the Internet has promoted diversification and increase of volume of information. In the mobile communication field, it has led to active research and development on next-generation wireless access systems for implementation of high-speed wireless communication. One of the next-generation wireless access systems is, for example, HSDPA (High Speed Downlink Packet Access). This HSDPA adopts the Adaptive Modulation and channel Coding (AMC) technology for varying the throughput according to a receiving environment of a mobile communication terminal. It is being studied in the HSDPA to equip a mobile communication terminal with a multipath interference canceller for canceling multipath interference, to enhance receiving performance of the mobile communication terminal and to improve the throughput.
Incidentally, a mobile communication terminal, when receiving a signal transmitted from a base station, also receives delayed waves. The delayed waves arise from reflection, scattering, and diffraction of the signal transmitted from the base station. Then the delayed waves cause multipath interference, so as to significantly affect the receiving performance of the mobile communication terminal. A method of reducing the influence of multipath interference due to the delayed waves is disclosed in the reference of A. Klein, “Data Detection Algorithms Specifically Designed for the Downlink of Mobile Radio Systems,” Proc. of EEEE VTC '97, pp. 203-207, Phoenix, May 1997. T. Kawamura, K. Higuchi, Y Kishiyama, and M. Sawahashi, “Comparison between multipath interference canceller and chip equalizer in HSDPA in multipath channel,” Proc. of IEEE VTC 2002, pp. 459-463, Birmingham, May 2002.
The multipath interference canceling method disclosed in the above reference will be described below with reference to FIGS. 8 to 10A and 10B. First, FIG. 8 shows a functional configuration of multipath interference canceller 90 used in this method. As shown in FIG. 8, the multipath interference canceller 90 has a delay profile creator 91, a channel matrix generator 92, a weight matrix generator 93, and an interference canceller 94.
The delay profile creator 91 despreads a signal transmitted through the use of a Common Pilot Channel (CPICH) from a base station, to generate a delay profile shown in FIG. 9. The horizontal axis of the delay profile shown in FIG. 9 represents time, and the vertical axis received power. The delay profile shown in FIG. 9 indicates that a desired wave SA arriving first has the largest received power SAP, a delayed wave SB arriving next has the smallest received power SBP, and a delayed wave SC arriving last has an intermediate received power SCP. p1 indicates a power difference between the received power SAP of the desired wave SA and the received power SBP of the delayed wave SB, and p2 a power difference between the received power SAP of the desired wave SA and the received power SCP of the delayed wave SC.
The delay profile creator 91 measures the received powers SAP-SCP and delay amounts d1, d2 of the respective received paths SA-SC, based on the delay profile shown in FIG. 9. It is noted that it is also possible to measure reception timings of the desired wave and delayed waves instead of the delay amounts. The delay profile creator 91 has an MF (Matched Filter) function.
The channel matrix generator 92 generates a channel matrix H shown in FIG. 10B, based on the number of taps (equalization window width) W and a maximum delay width D shown in FIG. 10A. The channel matrix H is expressed as a matrix of (W+D) rows and W columns. Here the number of taps W equals the number of samples for each path present in the maximum delay width D in generating the channel matrix H, and can be optionally set. FIG. 10A is a drawing showing only a portion corresponding to the lower part in the delay profile shown in FIG. 9.
The weight matrix generator 93 puts the channel matrix H shown in FIG. 10B, into mathematical expression 1 below to generate a weight matrix. In the mathematical expression 1 σ2 represents noise power and I a unit matrix.(ĤHĤ+σ2I)−1ĤH  [Mathematical Expression 1]
The interference canceller 94 multiplies data on channels by the weight matrix to cancel the multipath interference.
Incidentally, the multipath interference cancellation by the high-accuracy method as described above requires the matrix calculations in chip units, and signal processing according to the number of samples. Therefore, it increases the computational complexity in the mobile communication terminal and the processing imposes heavy loads on the mobile communication terminal equipped with the multipath interference canceller. Furthermore, the aforementioned AMC technology requires feedback, and it increases the computational complexity to cause excess of arithmetic processing time, which will result in a time lag and disturb operation in the mobile communication system.